Complexing agents are widely used in a variety of liquid media to form complex species or chelates with metals. Complexing agents are employed in metal cleaning solutions, metal etchants, electroplating baths and electroless metal deposition baths, as well as other liquid compositions.
By way of illustration, particular reference is made to electroless metal deposition, a well known means for forming adherent layers of metal on non-metallic surfaces. This generally involves treating a catalyzed surface with an electroless metal deposition bath, which is typically prepared by admixing water, an alkali pH adjuster, a water soluble salt of the metal to be deposited, such as the sulfate, chloride, nitrate, acetate, etc., a reducing agent for the metal, a complexing agent for the metal, and, usually, a wetting agent. Some examples of electroless metal deposition baths are disclosed, for instance, in U.S. Pat. Nos. 3,119,709 and 3,075,855.
It is known that bath life can be prolonged and bath performance can be maintained at desired levels of activity by replenishing from time to time bath ingredients which are consumed during electroless metal deposition. However, bath operation also results in the formation of by-products, e.g., alkali metal salts, which accumulate in the bath and eventually reach amounts that interfere with the electroless metallizing process. For example, the physical properties of deposited metals are inferior when high concentrations of such by-products are present in the bath and, in addition, maintenance of bath stability becomes more difficult.
Thus, when relatively large amounts of by-products are present in an electrical metal deposition bath, the bath usually must either be discharged or some adjustment must be made to the bath to enhance bath performance and to prolong its useful life. Adjustment is made, for instance, by diluting the bath with additional amounts of water to reduce the concentration of by-products in the bath, or by adding dilute amounts of consumable bath ingredients. Even with these adjustments, however, at least some of the bath must be disposed of, i.e., overflow resulting from additions to the bath.
Therefore, with any of the above methods, bath ingredients which are not normally used up, such as undeposited heavy metals and complexing agents, will be lost when the bath or its overflow are discharged. For example, in electroless copper baths anywhere from 5 percent to as much as 50 percent of the original amount of copper is not deposited. In addition, during the electroless metal deposition process, complexing agents are not consumed, but function merely as intermediaries in assisting the transfer of metal from the bath to the substrate being metallized. Thus, substantially all of the original amounts of complexing agent remains in the bath even after deposition is completed.
These ingredients, i.e., complexing agents and undeposited metals, are often expensive. It is desirable, therefore, to recover them for further use in electroless metal deposition baths. Moreover, conventional waste treatment systems are unsuitable for treating plant effluents which contain complexed heavy metals, which are found in discharges from electroless metallizing processes for instance. This is because waste treatment typically involves adjusting the pH of waste liquids to precipitate metals as insoluble salts, which are then filtered or allowed to settle. However, the presence of complexing agents for the metals usually prevents the formation of such precipitates during waste treatment.
Similar difficulties in waste treatment occur, of course, with other liquid media which contain complexing agents and complexed metal species, such as the above-mentioned metal cleaning solutions, etching solutions and electroplating baths. Although the use of these solutions normally does not involve the separation of by-products from complexed metals, as is desirable in electroless metal deposition, it is desirable for purposes of effective waste treatment and disposal to remove the complexed metals and complexing agents from such solutions prior to their dicharge into conventional waste treatment systems.
There has now been discovered a method which is broadly applicable to the separation of deposition by-products from complexed metals in electroless metal deposition operations, as well as to the waste treatment of solutions of complexed metals and complexing agents. More particularly, it has been surprisingly discovered that alkanolamine complexing agents and complex species of heavy metals and such agents can be removed from solutions by a method which involves pH adjustment and contacting with an ion exchange medium. This discovery is quite unexpected, in view of the fact that many other complexing agents can not be removed in any appreciable quantities under the same conditions.
Accordingly, it is an object of this invention to provide a method of operating an electroless metal deposition bath wherein alkanolamine complexing agents and heavy metals complexed therewith are separated from deposition by-products recovered in reusable form and, in continuously operating systems, returned to an electroless metal deposition bath for further use.
It is a further object of the invention to provide a method of treating solutions containing alkanolamine complexing agents and complexes of heavy metals and alkanolamine complexing agents to readily remove them from the solutions.
These and other objects of this invention will be apparent to those skilled in the art from the following description.